1) Field
Embodiments of the invention are in the field of X-ray reflectance scatterometry (XRS) and, in particular, methods and systems for measuring periodic structures using multi-angle XRS.
2) Description of Related Art
As integrated circuit (IC) features continue to be scaled to ever smaller dimensions, constraints on metrology used to measure such features become overwhelming. For example, critical dimension scanning electron microscopy (CD-SEM) metrology has several drawbacks that are becoming more significant with each new generation of IC technology. Drawbacks can include (1) the well known charging problem that limits the achievable resolution for IC metrology applications, (2) radiation damage induced dimensional shrinking of resists, (3) incompatibility with some low-k dielectrics, and (4) CD-SEM is essentially a surface technique making it difficult to measure three dimensional (3D) profiles.
Similarly, optical critical dimension (OCD) metrology faces a number of fundamental difficulties, including (1) the relatively long wavelength used is typically significantly larger than the device feature size and therefore does not provide a simple and direct measurement, and (2) OCD requires extensive modeling and interpolation, thus compromising the measurement sensitivity. Furthermore, over the last decades, use of shorter and shorter wavelengths has been necessitated by the reduction of circuit feature size. Currently the most advanced OCD system uses deep ultraviolet (DUV) wavelengths. Further incremental reduction in wavelength is not practical because of the extremely low transmission of shorter wavelength radiation in solids or even in low vacuum. Numerous problems can arise as a consequence, including low probing depth, lack of suitable optics, and stringent vacuum requirements. Such fundamental limitations have made it practically impossible to extend these existing technologies to meet the critical dimensional control requirements of next generation IC fabrication.
Grazing-incidence small-angle scattering (GISAS) is a scattering technique used to study nanostructured surfaces and thin films. The scattered probe is either photons (Grazing-incidence small-angle X-ray scattering, GISAXS) or neutrons (Grazing-incidence small-angle neutron scattering, GISANS). In either case, an incident beam strikes a sample under a small angle close to the critical angle of total external x-ray reflection. The intense reflected beam as well as the intense scattering in the incident plane are attenuated by a rod-shaped beam stop. The diffuse scattering from the sample is typically recorded with an area detector. However, since incident angles used in GISAS techniques is usually less than a few degrees, and even as small as a fraction of a degree. Accordingly, when used to measure 3D structures, information obtained through GISAS may be limited since the incident beam is directed mostly along only the top surfaces of such 3D structures.
Thus, advances are needed in metrology of 3D structures.